This invention relates to a positioner for controlling the location of a movable element, and, more specifically, to a positioner for controlling the location of a pointer image projected onto a viewing screen. Still more specifically, this invention relates to a positioner that moves a projected highlighting spot to commanded positions on a projection screen so as to highlight successively selected points in a pictorial image that is separately projected onto the screen. The positioner employs a servo loop that controls the pointer of a pointer image projector and thereby positions the pointer image as commanded. A pair of novel capacitive position sensors provide the requisite position feedback signals for the servo loop.
A variety of photographic image projection systems include pointers of one form or another that can be projected onto projection screens so as to point to or designate various parts of photographic images that are projected onto those screens. In this manner a lecturer, for example, can call an audience's attention to various aspects of a photographic image that he is discussing. One such system is described in U.S. Pat. No. 4,146,314 that is assigned to the assignee of this invention. That patent discloses several forms of automatic pointer apparatus for moving a highlighting spot over visual images in close coordination with an accompanying recorded audio program. The apparatus continually and rapidly generates a series of electrical signal sets. Each signal set corresponds to a two-dimensional position of the pointer image that is selected manually by an operator with a hand-held control device. The operator's movement of a control actuator, such as a joy stick, a stylus or even a finger, produces variations in the amplitudes of analog signals that form the signal sets.
Periodically the signal sets are sampled and converted into digital form and then recorded on a control track of a multiple track magnetic tape, along with control information that is also produced by the system. The recording of this information on the control track occurs simultaneously with the recording of the operator's spoken words, or some other audio program, on an audio track on the same tape. Concurrently the signals to be recorded on the tape are converted into suitable form for driving stepping motors to position a pointer mechanism that projects the pointer image onto the viewing screen. Thus, the pointer image follows the operator's movements of the control actuator.
During playback of the magnetic tape the recorded signals from the control track are converted into the form used for driving the stepping motors, so that the pointer image follows the signals recorded on the control track and thus retraces the path of the pointer image during the recording operation.
In the prior system there is a noticeable and disconcerting delay between the operator's movement of the control actuator and the positioning of the pointer image in response thereto. This lag forces the operator to concentrate on the pointer control function, and it therefore detracts from his concentration on the audio part of the program which he is simultaneously attempting to record. It can thus produce interruptions, pauses and repetitions in the audio program that degrade the overall effect of the program when it is played back.
Copending U.S. Pat. No. 4,291,954 discloses a system that effectively eliminates an undesirable lag between the operator's control movements and the movement of the pointer image in response thereto. In addition to that improvement, however, it is desirable to eliminate the discrete, jerky motions resulting from stepping motor movement of the pointer projector. The present invention is directed to the solution of this problem as well as to other improvements in the pointer movement mechanism.